Problem:

Op<TStart, Result<TRes, TErr>>.Chain(Op<TRes, TNewRes>)

Problem: not possible without .FlatMap because we might not have a TRes yet, so we can't just chain without early failure, so we need proper error handling.

Other benefits

Early cancellation, reporting in which step we failed

Considerations

Do not use fancy inheritance, like making that a subtype of Operation<TStart, Result<TRes, TErr>> to avoid confusion, complexity, edge cases and potential LSP violations